In order to form a phosphor layer (a phosphor screen) of a cathode ray tube, a so-called settling method has been widely adopted, in which phosphor particles in a suspension are settled on an inner screen surface of a main body of a cathode ray tube to form a sedimentary layer. The sedimentary layer of the phosphor particles is dried to obtain a phosphor layer.
A projection tube for a projection TV, a projection monitor and the like (a monochrome cathode ray tube) is usually used under conditions in which a plate voltage and an anode current are higher (15%-20% higher for the plate voltage and about five times higher for the anode current) than those of a direct viewing color cathode ray tube to compensate for a decrease in the quantity of light. Therefore, the phosphor layer on the inner screen surface receives a stronger stimulus, which accelerates the deterioration of the luminance remaining rate.
For improving such a deterioration, Japanese Laid-open Patent Publication No. 5-230455 discloses a phosphor layer formed out of a rare earth phosphor material including Lu (lutetium). Japanese Laid-open Patent Publication No. 5-263076 discloses a phosphor layer formed out of a rare earth phosphor material including Yb (ytterbium). Japanese Laid-open Patent Publication No. 5-302083 discloses a phosphor layer formed out of a rare earth phosphor material including Ce (cerium). Japanese Laid-open Patent Publication No. 63-279539 discloses a phosphor layer formed out of a phosphor slurry having a specific weight ratio of a phosphor material to a binder material.
One of the causes of the deterioration of a luminance remaining rate in a projection tube for a projection TV and the like is a change for the worse of luminance in the phosphor material itself included in the phosphor layer. Besides, in the settling method, since water glass (alkali silicate) is added to a phosphor layer as a binder from a phosphor suspension to increase the adhesive strength among phosphor particles, a change of color in silicon dioxide (SiO.sub.2) seriously causes the deterioration of a luminance remaining rate. Viewed in this light, SiO.sub.2 should be reduced as much as possible. However, when the water glass added from a phosphor suspension is made to decrease to reduce the SiO.sub.2 in a phosphor layer, the phosphor screen tends to become less fine because of a decrease of the adhesive strength among phosphor particles, thereby making screen quality worse. Such a problem can not be resolved by forming a phosphor layer out of a rare metal phosphor material including Lu, Yb or Ce, nor can the problem be resolved by forming a phosphor layer out of a phosphor slurry having a specific weight ratio of a phosphor material to a binder material.